Insert missing apostrophes in ctypes documentation

Doc/library/ctypes.rst

@@ -200,7 +200,7 @@ calls).
 Python objects that can directly be used as parameters in these function calls.
 ``None`` is passed as a C ``NULL`` pointer, bytes objects and strings are passed
 as pointer to the memory block that contains their data (:c:expr:`char *` or
-:c:expr:`wchar_t *`).  Python integers are passed as the platforms default C
+:c:expr:`wchar_t *`).  Python integers are passed as the platform's default C
 :c:expr:`int` type, their value is masked to fit into the C type.
 
 Before we move on calling functions with other parameter types, we have to learn
@@ -1441,7 +1441,7 @@ function exported by these libraries, and reacquired afterwards.
 All these classes can be instantiated by calling them with at least one
 argument, the pathname of the shared library.  If you have an existing handle to
 an already loaded shared library, it can be passed as the ``handle`` named
-parameter, otherwise the underlying platforms :c:func:`!dlopen` or
+parameter, otherwise the underlying platform's :c:func:`!dlopen` or
 :c:func:`!LoadLibrary` function is used to load the library into
 the process, and to get a handle to it.
 
@@ -1452,7 +1452,7 @@ configurable.
 
 The *use_errno* parameter, when set to true, enables a ctypes mechanism that
 allows accessing the system :data:`errno` error number in a safe way.
-:mod:`ctypes` maintains a thread-local copy of the systems :data:`errno`
+:mod:`ctypes` maintains a thread-local copy of the system's :data:`errno`
 variable; if you call foreign functions created with ``use_errno=True`` then the
 :data:`errno` value before the function call is swapped with the ctypes private
 copy, the same happens immediately after the function call.